In general, electrophotographic image forming devices such as copiers, printers and facsimile machines form a toner image on, or transfer a toner image to, the surface of an image bearing member such as a photosensitive drum or an intermediate transfer member, and after transferring the toner image, which was formed on, or transferred to, the surface of this image bearing member, to a recording media (such as copy paper or plastic film) the recording media is separated from the surface of the image bearing member by a separation means, the recording media that has been separated from the image bearing member is fed into a fixing means, and after the toner image is fixed on the recording media by this fixing means, the fixed recording media is transported to an output tray or an intermediate tray for two-sided printing.
Examples of known separation means for such image forming devices include charge removal separation systems such as shown in FIG. 8, charge removal separation systems such as shown in FIG. 9 and charge removal separation systems such as shown in FIG. 10. Therein, the separation means 100, 101 shown in FIG. 8 and FIG. 9 are such that separation needles 102, 103 are held by an upstream wall 104, 105, which is positioned upstream in the direction of transport of the recording media, and a downstream wall 106, 107, which is positioned downstream in the direction of transport of the recording media; a plurality of the separation needles 102, 103 being disposed orthogonal to the direction of transport of the recording media (hereinafter referred to as the paper travel transverse direction). Furthermore, the separation means 108 shown in FIG. 10 are such that a separation needle 110 having a plurality of discharge tips 110a at predetermined intervals in the lengthwise direction of an image bearing member 114 is disposed downstream from a transfer charger 109, in the direction of sheet transport, facing the image bearing member 114, so that when a voltage is applied to this separation needle 110, discharge current flows from the separation needle 110, so that the recording media 112 separates from the image bearing member 114.
Among these separation devices 100, 101 and 108 for image forming devices, the separation means 100 disclosed in FIG. 8 is such that, at a predetermined timing after transferring the toner image to the recording media 111, a separation voltage is applied to the separation needles 102, whereupon corona discharge is generated between the separation needles 102 and the image bearing member 113, so that the electrostatic charge applied to the recording media 111 by the transfer roller 117 during the transfer operation is removed, whereby the recording media 111 separates from the image bearing member 113 as a result of the weight and the elastic resilience of the image recording media 111.
Furthermore, with the separation means 101 disclosed in FIG. 9, excess charge is discharged to the grounded separation needle 103 from the back side of the recording media 115, so as to remove the electrostatic charge applied to the recording media 115 by the transfer roller 120 during the transfer operation. Thus, if the elastic resilience of the recording media 115 itself is strong, it is possible for charge removal separation between the recording media 115 and the image bearing member 116 to be performed well, even in cases where a conventionally known separation means 101 such as disclosed in FIG. 9 is used.
Furthermore, the separation means 108 disclosed in FIG. 10 are such that after a toner image on an image bearing member at 114 has been transferred to the recording media 112, the recording media 112 is separated from the image bearing member 114, but the conditions for separation at this time are the application of a predetermined voltage to the separation needle 110 and a predetermined gap size between the separation needle 110 and the image bearing member 114, whereby, when a transfer corona is generated by the transfer charger 109, the peripheral impedance changes and a discharge current can be observed from the separation needle 110, so that the recording media 112 separates well from the image bearing member 114.
It is of note that in recent years, with a view to efficient use of resources and protection of the environment, printing is often performed on both sides of the recording media. In such cases of printing on both sides of the recording media, there is a risk that the recording media 115 will not separate well from the image bearing member 116 with conventional separation means 101 such as shown in FIG. 9. Here, modes of printing on both sides of the recording media 115 include, not only two-sided printing wherein, after printing on one side of the recording media 115, the printing media 115 having one printed side is reversed to the back side using a two-sided printing unit or the like, and printing is continued on the other side of the printing media 115 which has not yet been printed, but also modes wherein, after printing on one side, the recording media 115 is reused as copy paper or the like, and printing is performed on the unprinted side of that reused printing media 115.
In other words, when such two-sided printing is performed, there is a tendency for the curl in the recording media 115, which is copy paper or the like, to be larger than the curl in the recording media 115 when only one-sided printing is performed, and in cases where this curl would tend to result in the recording media 115 wrapping onto the image bearing member 116, which is a photosensitive drum or the like (cases in which the recording media curls towards the image bearing body 116), the greater the curl, the less readily the recording media separates from the image bearing member 116, and there is a risk of the recording media 115 being wrapping onto the image bearing member 116 and causing problems such as jams.
Such problems can be effectively solved, for example, by applying a voltage of a polarity opposite to that of the transfer current to the separation needles 102, 110 in the separation means 100, 108 shown in FIG. 8 and FIG. 10, so as to generate a corona discharge between the separation needles 102, 110 and the image bearing member 113, 114, whereby the charge is removed from the recording media 111, 112 and the recording media 111, 112 separates more efficiently from the image bearing member 113, 114.
However, with these separation means 100 as shown in FIG. 8, if the upstream wall 104 and the downstream wall 106 are formed from an insulating resin, the upstream wall 104 and the downstream wall 106, which are made from the insulating resin, are charged by the corona discharge generated between the separation needles 102 and the image bearing body 113, as a result of which the electrical fields surrounding the plurality of separation needles 102 (the electrical fields in the paper travel transverse direction) are not uniform, but rather depend on the position thereof in the paper travel transverse direction. At this point, if an attempt is made to produce the desired separation function with the recording media 111 by increasing the voltage applied to the separation needles 102, there is a risk of abnormal discharge between the separation needles 102 and the image bearing member 113, resulting in damage to the insulation of the image bearing member 113, which is a photosensitive drum or the like.
Likewise, with the separation means 108 as shown in FIG. 10, when predetermined values are used for the gap between the separation needle 110 and the image bearing member 114 and the like, so as to generate a discharge current, separation conditions vary depending on the voltage that is applied to the transfer charger 109 and the separation needle 110, which is unstable.
Here, the present invention provides an image forming device in which damage to insulation on the image bearing member is prevented by decreasing the discharge voltage by way of increasing the discharge efficiency of the separation means, and by improving the uniformity of the discharge from the separation means in the paper travel transverse direction, and the recording media separates well from the image bearing body.